Stretch forming apparatus and method



V. BENATAR ETAL STRETCH FORMING APPARATUS AND METHOD Filed Nov. 5. 1962"1\\\\\\\\\\\\\\\\\\\\\ l `\s\\"`\\\\\\\` l I ,I /fkf 4l March s, 1966Fl G. l

INVENToRs. VICTOR BENATAR IRA H. CARPENTER Agent FIG. 2

United States Patent 3 238 753 s TRETCH roRMING PPRATUS AND METHODVictor Benatar and Ira H. Carpenter, Atlanta, Ga., as-

garlritfirs to Lockheed Aircraft Corporation, Burbank,

Filed Nov. 5, 1962, Ser. No. 235,446 5 Claims. (Cl. 72-56) Thisinvention relates to the forming of sheet and/or plate metals, and moreparticularly to a method and apparatus for the high energy stretchforming of metal parts from sheet and/or plate forms.

In recent years the aerospace industry has seriously sought to developnew economic methods and apparatus for the forming of parts from sheetand plate metals. Mechanical devices such as the hydropress and stretchform have been used in the past and are still being used 4for manyforming operations. However, it has been found to be uneconornical touse such mechanica-l devices in the forming of high strength metals suchas AISI type 302 stainless steel, for example, because of the inabilityof such devices to generate work forces suflciently great to form themetal without resort to massive and costly new presses to do the job.Such mechanical devices have further limitations as to the physical sizeof the work pieces they can form and it is not economical to use themunless on a mass-production scale.

To eliminate the disadvantages and limitations of such prior artmechanical devices, extensive work has been done in developing explosiveforming techniques for forming metal work pieces. Toward that end,methods and apparatus have been ydeveloped for the explosive forming ofmetal work pieces which have met with considerable success. Onelimitation of such explosive forming devices, however, is that they arenot well-suited for the forming of shallow contoured parts to closetolerance; and the forming of shallow contoured parts by such deviceshas resulted in a substantial number of rejected parts. In experimentingand working with explosive forming of -shallow contoured parts, it wasdetermined that the inability to form parts to close tolerance wascaused by spring back of the part after instantaneously taking theconfiguration of the die and that spring back was caused by a failure ofprior art die structures to provide a space or area to permit theelongation of the work pieces in their plastic range so that they wouldset into the cont-our of the die, the shallow contoured die in etfectbacking the work piece so as to prevent such elongation of the workpiece. It was further found that the use of presently known highvelocity explosive forming techniques in combination with evacuation ofthe space between the work piece and the die, although effective inreducing spring back did not eliminate it from consideration in theforming of shallow contoured parts to close tolerance. Accordingly, thisinvention is directed at improving upon explosive forming techniques andthe extension of their use to the for-rning of parts of shallow contourto close tolerance. Toward this end there is provided method andapparatus for explosive stretch forming and setting a work piece intothe configuration of a shallow contoured die with closer tolerance thanheretofore obtainable, this objective being achieved by the provision ofdie structure which permits the elongation of the work piece throughoutits area in its plastic lrange so that it will be formed or set to theconfiguration of the die.

Accordingly, it is an object of this invention to provide an apparatusand method for forming metal work pieces into shallow contoured partswithout large and heavy forming device installations.

Another object of this invention is .to provide a method 3,238,753Patented Mar. 8, 1966 ICC and apparatus for the explosive stretchforming of large integral shallow contoured parts to close tolerance.

It is a further object of this invention to provide an apparatus andmethod for forming metal work pieces into shallow contoured parts withclose tolerances without concern to spring back.

A still further object of this invention is to provide an apparatus andmethod for the high energy stretch forming of a work piece to theconfiguration of a shallow contoured die.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawings in which:

FIGURE l is a sectional side view of one embodiment of the high energystretch forming apparatus of this invention immersed in an energytransfer medium; and

FIGURE 2 is a top plan View of the explosive forming die of FIGURE l.

Generally stated, .the invention is practiced by forming a metal workpiece through utilization of the dynamic for-ces of pressure wavesgenerated by the detonation of an energy source or explosive medium suchas high explosives, chemical reaction, etc., and directing the pressureWaves through an energy transfer medium against .the work piece in sucha manner as to form it to the configuration of a shallow, simple o-rcompound, contoured male form die. In accordance with this invention,the shallow contoured die is constructed so as to permit the stretchingor elongation of the metal being formed throughout its area in theplastic range so that it will set to the configuration of .the die.Toward this end, a shallow contoured male form die is provided with aperipheral stretch groove and the work piece is elongated or stretchedinto the stretch groove such that the work piece is elongated sufiicientthat its plastic range is achieved and the work piece is drawn andstretched into .the configuration of the die. Either low or highvelocity forming techniques may be employed; however, high velocityforming is preferred because of the advantages attendant its use.

Referring now to the drawings, there is shown in FIGURE 1 a shallowcontour male form die 5 which has been depicted as being of sim-plecontour, mounted in a cont-ainer 6, and covered with the energy transfermedium 7, preferably water. The male form die 5 has an upper formingsurface 10 of shallow contour in which is for-med a preferablycontinuous peripheral stretch groove 11. The stretch groove 11 isconnected by a plurality of openings 12 which are connected by line 13to a vacuum system, not shown. Disposed in concentric surroundingrelation to the groove 11 there is provided an annular seal 14 fittedinto a groove 15 to hold it in place, and disposed outside the seal 14are a plurality of clamping assemblies 17 which clamp the Work piece 20into sealing engagement with the seal 14 and prevent movement of thework piece outer edge during the forming operation. Also, at least oneor more energy sources, explosive medium or charges 8 are disposed inthe energy transfer medium 7 above the work piece 2i) in the vicinity ofstretch groove 11 such that the pressure waves formed by theirdetonation will be equally distributed over the area of the work piececovering the stretch groove 11.

In operation, a metal work piece or part 20 is placed on the upperforming surface 10 of the male form die 5 and is clamped in place withits peripheral edge 20a firmly fixedly clamped into sealing engagementwith the seal 14 by the clamp assemblies 17. The die assembly 5 is thenplaced in the container 6 within the energy transfer medium 7; explosivemedium or charges 8 are placed above the work piece 20 in the vicinityof the stretch groove; and the air entrapped in the passage 12, thegroove 11 and between the work piece 20 and the surface 10 is evacuatedby the vacuum system such that the entrapped air will not cushion theexplosive force and prevent forming of the work piece into the shallowcontour of the forming surface 10. After the air has been evacuated frombetween the work piece 20 and the die 5, the explosive medium 8 aredetonated and upon detonation generate pressure waves which aretransmitted through the energy transfer medium 7 at least over the areaof the work piece 20 covering the stretch groove 11 such that the workpiece 2t) covering the stretch groove 11 is stretched and elongated inits plastic range into the stretch groove 11 and in so doing draws,stretches or elongates the remaining portion of the work piece 20 in itsplastic range into the shallow contour of the top surface 10 of the dieso that the work piece 20 will retain the configuration of the topforming surface 10.

It is important to take particular note at this point that for the workpiece 20 to retain the shallow contour of the u die forming surface 10,it must be elongated or stretched such that it undergoes plasticdeformation throughout its area extent circumscribed by the stretchgroove 11, which may properly be called the network piece or part,Without exceeding the ultimate strength of the metal work piece and thatthe die structure comprising stretch groove 11 provides a suiiicientarea whereby this elongation or stretching of the net part may occur.

After forming and setting the work piece 20 into the shallow contour ofthe top surface 10 of the die, the die 5 is removed from the energytransfer medium 7 or the energy transfer medium 7 is removed fromsurrounding relation to the die 5, as would be preferable if a large diewere used, and the work piece 20 is removed from the die. Thereupon thework piece 20 may be cut along the line 20h leaving the tinal shallowcontoured net part.

It will be seen from the above that the apparatus and method of thisinvention provides for the high energy forming of materials into shallowcontoured parts of close `tolerance without concern for spring back, andthat such is made possible by the provision of the stretch groove aroundthe periphery of the male form die so that the work piece can bestretched or elongated in tension sufficient that it is stretchedthroughout in its plastic range.

By way of emphasizing the importance and value of the explosive stretchforming method and apparatus of this invention, it should be pointed outthat such method and apparatus makes possible for the first time theforming of large aircraft fuselage skin sections and large tank domegore segments of space craft of integral construction and shallowcontour within close tolerances without resort to mechanical formingdevices. This construction has the important advantages of eliminatingor minimizing welding and heat treating which results in degradation ofthe structural properties of the parts in the weld area and eliminatesor minimizes costly chemical milling of such parts to reduce theirthickness, except in the weld zone, to minimize the weight penaltyattendant yto welding.

While a particular embodiment of the invention has been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchmodifications and equivalents as fall within the true spirit and scopeof this invention.

What is claimed is:

1. A method of stretch forming a piece of sheet metal into a shallowcontoured work piece over a male die of shallow contoured configurationhaving a peripheral stretch groove, said stretch formed work piecehaving trim stock around the periphery thereof with a groove formed fromthe die stretch goove located in the trim stock portion of the sheetmetal outside of the periphery of the final desired arca of the stretchformed work piece comprising the steps of: securing the edge of thesheet metal piece to the die in covering relation to the stretch groove;placing explosive medium above the sheet metal such that pressure wavesgenerated by its detonation will impinge upon and be distributed overthe area of the sheet metal covering the stretch groove; and detonatingthe explosive medium to stretch the sheet metal into the stretch groovewhereby the portion of the sheet metal located within the area boundedby the die stretch groove to constitute the work piece is stretched inits plastic range and drawn and set into the configuration of the die.

2. A method of stretch forming a piece of sheet metal into a shallowcontoured work piece having trim stock around the periphery thereofcomprising the steps of: providing shallow contoured male die structurehaving a peripheral stretch groove external to the area of the finalconfigured work piece and within the area of the work piece trim stock;evacuating the space between the sheet metal and the die; placingexplosive medium above the sheet metal in the vicinity of the die; anddetonating the explosive medium to generate pressure Waves thereby tostretch the sheet metal into the groove whereby the portion of the sheetmetal located within the area bounded by the die stretch groove toconstitute the work piece is stretched in its plastic range and is drawnand set into the shallow contour of the die.

3. A method of stretch forming a piece of sheet metal into a shallowcontoured work piece having trim stock around the periphery thereofcomprising the steps of: providing shallow contoured male die structurehaving a peripheral stretch groove external to the area of the iinalconfigured work piece and within the area of the work piece trim stock;evacuating the space between the sheet metal and the die; placing thedie in an energy transfer medium; placing explosive medium above thesheet metal in the vicinity of the die; and detonating the explosivemedium to generate pressure waves thereby to stretch the sheet metalinto the goove whereby the remaining portion of the sheet metalcircumscribed by the stretch groove is stretched in its plastic rangeand is drawn and set into the shallow contour of the die.

4. For use in the explosive stretch forming of metal sheet to workpieces of shallow contour, a male form die assembly comprising: a maleform die having a shallow contoured upper forming surface, the area ofsaid upper forming surface greater than the area of the formed workpiece; a stretch groove formed in said upper forming surface external tothe area of the forming surface used to form the work piece and about atleast a portion of the work piece forming area of said forming surface;and clamping apparatus disposed outside said stretch groove for iixedlyclamping a piece of sheet metal to said forming surface in coveringrelation to said stretch groove.

5. For use in the explosive stretch forming of metal sheet to workpieces of shallow contour, a male form die assembly comprising: a maleform die having a shallow contoured upper forming surface, the area ofsaid upper forming surface greater than the area of the formed workpiece; a stretch groove formed in said upper forming surface external tothe area of the forming surface used to form the work piece and aboutthe periphery of the work piece forming area of said forming surface; avacuum system connected to said stretch groove; a seal mounted insurrounding relation to said stretch groove; and clamping apparatusdisposed outside said stretch groove for ixedly clamping a piece ofsheet metal to said forming surface in sealing relation with respect tosaid seal and in covering relation to said stretch groove.

References Cited by the Examiner UNITED STATES PATENTS 3,065,720 11/1962Rardin 113-44 FOREIGN PATENTS 115,846 2/1946 Sweden.

CHARLES W. LANHAM, Primary Examiner.

1. A METHOF OF STRETCH FORMING A PIECE OF SHEET METAL INTO A SHALLOWCONTOURED WORK PIECE OVER A MALE DIE OF SHALLOW CONTOURED CONFIGURATIONHAVING A PERIPHERAL STRETCH GROOVE, SAID STRETCH FORMED WORK PIECEHAVING TRIM STOCK AROUND THE PERIPHERY THEREOF WITH A GROOVE FORMED FROMTHE DIE STRETCH GOOVE LOCATED IN THE TRIM STOCK PORTION OF THE SHEETMETAL OUTSIDE OF THE PERIPHERY OF THE FINAL DESIRED AREA OF THE STRETCHFORMED WORK PIECE COMPRISING THE STEPS OF: SECURING THE EDGE OF THESHEET METAL PIECE TO THE DIE IN COVERING RELATION TO THE STRETCH GROOVE;PLACING EXPLOSIVE MEDIUM ABOVE THE SHEET METAL SUCH THAT PRESSURE WAVESGENERATED BY ITS DETONATION WILL IMPINGE UPON AND BE DISTRIBUTED OVERTHE AREA OF THE